1. Field of the Invention
The present invention relates to a negative differential resistance (NDR) device and memory using the same.
2. Description of Prior Art
A negative differential resistance (NDR) device has the characteristic of negative differential resistance on its current-versus-voltage curve. Namely, the current is decreasing while the voltage is increasing. For example, resonant tunneling diode, tunneling diode, Gunn diode, resonant tunneling transistor and so on are the most common negative differential resistance devices.
NDR device has great potential in design and application of RF circuits because NDR device has the advantages of fast switching and operating in RF area (the working frequency is up to 1 GHz). Moreover, frequency multiplier, oscillator, multiple-state memory, analog/digital converter, current-and-voltage-level reference, multiple-value counter, multiple-value multiplexer, logic circuit, pulse generator have promising applications based on the NDR device.
However, previous NDR devices have three primary features hindering their application. First, the processes involved are usually not easy to control or even too complicated to be compatible with Si technology. Second, the peak-to-valley current ratio (PVCR) is usually too small, not high enough for memory applications considering both the switching speed and power consumption. Third, the value of current of those devices might depend on some sensitive factors (tunneling current, trapping effect, etc., for examples), which makes the uniformity of the fabrication becomes very critical and hard to control.
Random Access Memory (RAM) is a kind of volatile memory, which includes both Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM). DRAM requires periodical refresh to preserve the data while SRAM preserves data due to self-latching effect. Due to its self-latching feature, the switching speed of SRAM is fast among other semiconductor memory and is used widely nowadays. Typical SRAM is consisted of six transistors and can hold the data even after the write/read operation since the cross coupled inverters formed a negative feedback to latch the data. Although SRAM is faster than DRAM, the price is higher since it needs more area than a typical one-transistor DRAM.
Since NDR device has bi-stable feature, it can be also applied to design memory. For example, SRAM can be implemented by NDR device in prior art reference. However, the PVCR thereof is not high enough (higher standby power or slower speed), the manufacturing process is complicated and the result-in SRAM might still occupy considerable area.